Fixes to Game startup when using a unit test runner.
The Content.RootDirectory cannot be absolute.
Removed local directory reference.
Using AppDomain.CurrentDomain.BaseDirectory seems to be a more reliable way of getting the working dir.
I was wrong.... Content.RootDirectory can have an absolute path.  MonoGame needs fixing here.
Game takes care of disposing GraphicsDeviceManager.
Fixed the Matrix.CreateBillboard function so that both overloads work.
Switched overloads for efficiency.
Fixed keyboard mapping for linux to include the right shift key, the right windows key, and the ? key.
Merge pull request #1546 from AdamDawes/FixForMatrixCreateBillboard

Fixed the Matrix.CreateBillboard function so that both overloads work.
Implemented missing SoundEffect.IsDisposed
Merge pull request #1547 from theodis/develop

Fixed missing keyboard mapping for Linux
Merge pull request #1560 from dellis1972/soundeffectdisposed

Implemented missing SoundEffect.IsDisposed
Addition of the MathHelper unit test. More tests are needed.
Merge pull request #1530 from SickheadGames/testfix

Unit Test Fixes
Merge pull request #1562 from totallyeviljake/MATH_HELPER_UNIT_TEST

Addition of the MathHelper unit test. More tests are needed.